1. L. Landau and E. Lifshitz,The Classical Theory of Fields(Addison-Wesley Press, Inc., Cambridge, Massachusetts, 1951), pp. 213–218.
2. B. A. Trubnikov and V. S. Kudryavtsev,Proceedings of the Second United Nations Conference on the Peaceful Uses of Atomic Energy, Geneva, 1958 (United Nations, Geneva, 1958), Vol. 31, p. 93.